Conventional heating elements are made from three traditional groups of materials comprised of (1) refractory metals like Mo and W; (2) resistance heating alloys including Fe—Cr—Al (Kanthal) and Ni—Cr (nichrome); and (3) high temperature, electrically conductive metal carbides and silicides including SiC and MoSi2.
Many devices employ metal-based resistive heater elements. These metallic films, foils, and filaments are formed or deposited with a variety of conventional techniques that include wire drawing, screen printing of inks, pastes and slurries, sputtering, electron beam deposition, physical vapor deposition (PVD), chemical vapor deposition (CVD), and arc plasma (flame) spraying.
U.S. Pat. No. 5,881,208 by Geyling, et al., discloses an apparatus for rapid thermal processing (RTP) of silicon wafers. The apparatus may include a heating element, sensing element and a cooling element, all of which are positioned in opposing relation to a backside of a silicon wafer. The heater element may be, for example, a resistance heating element with a CVD coating of pyrolytic graphite on a pyrolytic boron-nitride (PBN) substrate, such as the Boralectric line of heaters available from Advanced Ceramics Corporation. Metallic conductor layers may also be deposited by patterned CVD. The graphite is then machined to form a serpentine pattern that is then used as a resistive heating element. One serious disadvantage with CVD and chemical vapor infiltration techniques is that they frequently rely on the use of extremely toxic and explosive vapors such as silanes (methyl trichlorosilane) and hydrogen gas in low-pressure vacuum deposition chambers. Another significant disadvantage is that deposition rates are relatively slow, and thicker films will require excessively long times to fabricate using typical CVD processes. The operational and safety precautions that must be used with CVD techniques are often very expensive, time consuming and cumbersome.
Various methods for producing lithographically defined heating elements are commonly practiced and known to those skilled in the art. Development of thin film materials conventionally involves subjecting Si wafers or other substrates to a sequence of deposition, lithographic, and post deposition process steps. Process sequences are constrained and limited by the lowest maximum temperature tolerance of any other device or layer on the substrate being processed. Process steps involve mounting the sample in a chamber with controlled environments and successive runs through cycles that include high vacuum, e-beam, furnace treatments, sputtering, chemical vapor deposition (CVD), ion milling, and so on. R. E. Cavicchi, et al., “Pulsed desorption kinetics using micromachined microhotplate arrays”, J. Vac. Sci. Technol. A, vol. 12, p. 2549, 1994 and others have developed Si-based micro-machining techniques for fabricating “micro-hotplates” that may be useful as gas sensors and components in micro-chemical reactors or “lab-on-a-chip-devices”, (see for example “Miniature reaction chamber and devices incorporating same”, U.S. Pat. No. 6,284,525 by Mathies, et al., and U.S. Pat. No. 6,337,212 by Nagle, et al.) The maximum operating temperature of micro-heaters fabricated from refractory metals such as tungsten (W) appears to be about 800 C in non-oxidizing environments (usually accomplished by coating or burying the heating element in an inert layer). Aluminum micro-hotplates can be operated at temperatures up to about 500 C.
While it is possible to create small (<100 micrometers), fast reacting heating elements with micro-machining techniques, it is also well known that these processes are time and labor-intensive, and require very large capital and operational expenditures. In general, widespread usage has been limited by the cost of complicated micro-fabrication processes, use temperature restrictions, and wet chemical processing requirements.
U.S. Pat. No. 6,341,954 by Godwin, et al., discloses a molding system using film heaters and sensors. Preferably, the thin film element comprises a thin film heater in direct contact with the molten resin and is positioned to aid in the heat and flow management of the resin within the melt channel. Tungsten, molybdenum, gold, platinum, copper, TiC, TiCN, TiAlN, CrN, palladium, iridium, silver, and conductive inks are the recommended materials for resistive heating films. Ion plating, sputtering, chemical vapor deposition (CVD), physical vapor deposition (PVD), and flame spraying are recommended as deposition technologies for fabricating the heating films. Heating requirements (or watt densities) are in the range of 6.2–12.4 W/cm2.
U.S. Pat. No. 5,616,266 by Cooper describes methods and compositions for constructing thin film electrically conductive resistance heating elements. According to the teaching of this patent, vapor deposition of an area thin film in the form of a tin oxide film of about 3000 to 5000 angstroms is most preferred, however other film thickness and materials can be employed, as are well known in the industry and set forth in the patents. When operating under air-radiant conditions, as in an oven application, the substrate and film have an area sufficiently large to cause the heating element to operate at a power density of less than about 1.55 W/cm2 at the maximum operating temperature of the heating element, however it is claimed higher watt densities (up to 23.25 W/cm2 maximum) can be attained when liquids or gases flowing inside a tubular substrate absorb heat from the substrate. Tubular shaped elements have the added complications and expense of requiring apparatus to rotate the element during deposition or sputtering of the conductive material onto the tubular substrate.
“Thick” and “thin” film resistive heating elements have many inherent disadvantages that generally limit their usefulness and applicability. One problem with the film deposition processes is that voids or pinholes can be formed in the film materials during sequential deposition or printing and firing steps. Another more serious disadvantage is that the requirement for building up many multiple film layers in order to create more durable or complex element features results in an expensive process due to the increased time and number of individual process steps that are involved. (For example, the recommended process for building up thick film heating elements typically involves multiple applications of an underglaze, fabrication of a print screen, application of an emulsion to the print screen, coating the print screen with a metallic thick film ink, multiple printing, controlled drying and firing steps that require precise alignment to build up the thickness of the resistive heating film, followed by printing, drying and firing on low resistivity silver contact layers, often followed by printing, drying and firing on protective insulating overglaze layers.) Yet another common problem with “thick” and “thin” film heating elements is that they allow only a relatively low surface-specific heating power (usually less than 2.34–3.10 W/cm2), and only relatively low radiant surface temperatures (usually less than 500–600 C maximum). Typically, the film must be thin enough to insure “molecular bonding” with the substrate.
Techniques that involve painting, spraying and particularly screen printing metallic conductors in the form of a paste or ink onto a substrate are also in widespread use. One manufacturer, Dupont Electronic Materials, produces screen printable conductive inks under the trade name of Heatel® that are designed for the fabrication of thick film resistive heating elements. Thick films with an emulsion build-up of about 10–12 micrometers and print speeds of 100 to 150 mm/sec. are possible. The printed films must be dried and then sintered at a maximum temperature of about 850 C for about 30 minutes. In one application, the printed “thick film” Heatel® heating elements have been used to replace traditional (nichrome wire) resistance coils in appliances such as electric kettles, and are said to offer fast-boil capability. Another manufacturer, Electro-Science Laboratories, produces a screen printable silver palladium conductor with a firing range between 625 C and 930 C. The emulsion thickness is about 25 micrometers with a −325 mesh screen, and the fired thickness of the film is about 12 micrometers. The paste has a shelf life of only 6 months, and the resistivity is in the range of about 2 to 8 milliohms/square.
A particular disadvantage of printable conductors in the form pastes and inks is that the materials tend to be very expensive. Metallization systems such as Pd/Ag, that are most commonly used to print resistive heating films can cost $500.00 or more for quantities as small as 50 grams, making anything but relatively thin films (that is less than about 50 micrometers) impractical from the cost standpoint alone. Most Conductive pastes and inks will also contain many volatile solvents and other environmentally undesirable constituents that are used to adjust the rheology of the paste. Conductive pastes and inks that are frequently used for resistive heating applications at lower temperatures (about 300–500 C maximum) are Pd/Ag, Pt/Ag, Pt/Au, and Pt. A disadvantage inherent with screen printable inks and pastes is that they often require multiple print, dry, and sinter cycles to build-up sufficient thickness for applications that require higher power and heat transfer capabilities. These disadvantageous multiple cycles increase the manufacturing costs. In order to be compatible with Low-temperature Co-fired Ceramic tape (LTCC) systems these low-temperature conductive inks and pastes are designed to be fired at temperatures of no more than about 850 C. Conductive pastes and inks must be carefully formulated and adjusted in order to match the thermal expansion coefficient (TCE) of the substrate, and the large shrinkage rates in the case of co-fired (LTCC) ceramic systems. Much effort has been directed at solving problems with the TCE mismatch and shrinkage that can result in poor adhesion, buckling and warping of metallic “thick” and “thin” films on various substrates.
Several U.S. patents, U.S. Pat. No. 5,550,526 by Mottahed for example, describe techniques for forming recesses or cavities in multilayer devices that serve to thermally isolate radiating or heating elements. For example, the substrate beneath the isolating layer is generally etched away so that a suspended membrane is formed upon which the heater element is located, thereby increasing the thermal isolation of the heater element, which results in a significant decrease in power consumption. This is important for portable sensing instruments which are reliant on battery power. Thermal isolation profoundly effects the response times, power consumption and efficiency of heaters and thermally radiating elements.
As is well known to those skilled in the art, one of the most challenging and persistent problems with “green” or pre-fused ceramic tapes and multilayer lamination technologies has been devising practical methods to successfully eliminate slumping or deformation of suspended or laminated structures during the high temperature sintering process. The sagging or slumping problem is particularly exacerbated as the length of the suspended laminate increases, (see for example H. Bau, et al., Ceramic Tape-Based Meso Systems Technology, Proceedings of the ASME International Mechanical Engineering Congress and Exposition, Anaheim, Calif. 1998). Also, serious deformation can occur when stacks of green ceramic tapes are laminated under even relatively low pressures to form monoliths containing structures like channels or cavities, particularly when these structures have high aspect ratios. The three main strategies commonly used to control sagging are (1) the deposition of thick films to exert tensile forces on the bridging layer, (2) the use of sacrificial materials (lead bi-silicate glass frit), and (3) the use of fugitive pastes (graphitic carbon black paste), (see for example P. Espinoza-Vallejos, et al., The Measurement and Control of Sagging in Meso Electromechanical LTCC Structures and Systems”, MRS Conference Proceedings, Vol. 518, 1998). All of the previously developed anti-sagging techniques appear to be limited in their usefulness and success. For example, the tensile force that the bridging thick film exerts must be carefully balanced with the force that is inducing the sagging of the suspended layer, and the extra complication of depositing a thick film adds time, and expense to the process. Also, lead bi-silicate frits and other sacrificial materials often leave a residue that requires etching in hydrofluoric acid (HF) or other undesirable processes that may damage structures to remove the residue. Finally, the fugitive carbon pastes and most other solvent-based pastes are susceptible to drying, shrinking, cracking or deforming, multiple pastes layers are desired to build up sufficient thickness for many applications, and the pastes are inconvenient to apply in a rapid precise manner, all of which increases the desired time, labor and overall fabrication costs.
Many important resistive heater applications exist for materials that can operate at temperatures greater than about 500 C in air or corrosive atmospheres for prolonged times. Under some circumstances it may be possible to deposit thick films of refractory metals such as W, Mo, Ta, Pt and Pd deposited on high-temperature ceramic materials such as aluminum oxide or zirconium oxide, however nearly all metals (including W, Mo, and Ta) are unsuitable for use in air or corrosive atmospheres because they degrade rapidly (oxidize or corrode) at higher temperatures, and the inert Pt group metals (Pt, Ir, Rh, Re, Pd) are all prohibitively expensive. Another major disadvantage that is common to nearly all metal-based heating elements (including platinum group metals) is that the intrinsic bulk resistivity of metals is far too low, (usually less than 1 milliohm·cm), to achieve high power densities and surface temperatures. Due to the low intrinsic resistivity of most metals, the resistive heater designer is forced to use very thin, long meandering films or filaments in order to increase the resistance of the element. Unfortunately, the durability and power handling capability decrease rapidly as the cross sectional area of the metallic conductor decreases, and defects (such as a void or variations in the thickness) become more critical. Oxidized metals tend to be brittle, and the oxidized layer may not be adherent to the underlying material, leading to mechanical failure. Also transient electrical current surges, overvoltages, and hot spots can easily develop that lead to melting of metallic elements, as is well known in the resistive heating industries.
An ever growing need for higher process temperatures and better performance in oxidizing or hostile environments has led to the development of non-metallic resistive heating materials. In particular, SiC, MoSi2 and certain other non-oxide ceramic materials in the carbide, silicide, and boride groups have proven to be effective as high-temperature (>1200 C) resistive heating elements. With the addition of dopants and insulating filler materials, a much wider range of resistivities can be achieved with electro-conductive metal-carbides and silicides such as SiC or MoSi2, and this allows for higher power densities with greater reliability.
A number of prior patents discuss methods for producing hot-pressed, SiC or MoSi2-based, electro-conductive ceramic heating elements. For example, U.S. Pat. No. 5,514,630 by Wilkens, et al., discloses ceramic resistance igniters that are useful as compact radiant energy sources for replacing pilot lights in liquid and gas fuel burning appliances. Ceramic powder blends consisting primarily of MoSi2, SiC, Al2O3, and AlN are loaded into a hot press and hot pressed to form a billet of about 60% theoretical density. The billet is then green machined to form tiles that are subsequently subjected to high temperature hot isostatic pressing (HIP) at 1790 C and 30,000 psi for 1 hour. After HIP the dense tiles are diamond machined to form a “hairpin” design igniter. The ceramic igniters are claimed to have sufficiently high resistivity (0.3 ohm·cm at 1300 C), a satisfactory time-to-temperature (4 seconds to 1100 C), and electro-thermally stability up to 132 Volts.
Unfortunately, the electro-conductive ceramic igniters tend to be mechanically fragile and fracture easily due to the fact that SiC (and nearly all other monolithic ceramic materials) are very brittle, and stress is concentrated at the sharp bends in the unsupported “hairpin” design of the igniters and heating elements. Although SiC, MoSi2, and related carbide and silicide ceramics have many of the properties that make them near ideal for high temperature reisitive heating applications, they are difficult to process and it is impractical or nearly impossible to produce shapes that are more complex than simple cylinders and plates without expensive diamond machining and grinding operations. Dense ceramic carbides, borides and silicides typically require expensive or exotic process equipment including hot isostatic presses (HIP), capable of reaching temperatures of 1700 C and pressures of 30,000 psi or more under inert or evacuated atmospheres. The HIP process is energy intensive and time consuming, and is therefore not economical or practical for high volume production applications or where more complex shapes are desired.
U.S. Pat. No. 5,252,809 by Denim discloses a panel heating element that utilizes an electrically conductive heating layer applied laminarly to a non-conductive support element. The heating layer is applied in a pasty state and consists of powdered molybdenum borides (MO2B5, MoB, Mo2B) and two different powdered types of glass and, preferably, an organic substance such as cyclohexanol, lanolin or petrolatum oil. According to the Denim patent, it is especially important that the first type of glass has a softening temperature not exceeding 700 C. Molybdenum boride in the panel heating element according to the invention is the material that gives the heating layer the electrical conductivity. Since the first type of glass begins to soften starting from 700 C the molybdenum boride is protected from an undesirable air supply. So that the first type of glass does not completely melt with increasing temperature, a protective skeleton is incorporated by the second type of glass. The heating layer, and optionally the insulating layers, are applied in pasty state by a silk screening process. Afterwards the heating layer, and optionally the insulating layers, are baked at a temperature of about 820 to 875 C. The authors claim that surface-specific heating powers of up to 13 W/cm2 and a heating temperature of 800 C are possible according to the invention. Most known panel heating elements, including the Denim invention, provide a relatively low surface-specific heating power and only relatively low heating temperatures, and they are relatively expensive labor intensive processes that require multiple printing, drying, and sintering stages.
U.S. Pat. Nos. 5,565,387 and 5,420,399 by Sekhar, et al., disclose an electrical heating element and compositions for producing such heating elements using a method described as dieless micropyretic synthesis. The compositions include a filler material, a reactive system comprised of at least two particulate combustible materials capable of undergoing micropyretic synthesis, and (optionally) a plasticizer or extrusion agent. The filler materials specifically mentioned in the claims are SiC, MoSi2, Fe, Cr, Al, Si, Y2O3, Al2O3, SiO2, ZrO2, MgO, Si3N4 and small amounts of B or BN(<1%). Filler materials are said to have three important effects on the products, (1) the filler materials tend to moderate the combustion process and may be used to impart properties not necessarily present in the combustion synthesized products, (2) they act as reinforcements in the final composition products, and (3) fillers may act as sintering aids. The Sekhar, et al., invention relies on micropyretic synthesis or Self-propagating High-temperature Synthesis (SHS) to make the desired product. At least two of the materials in the reactive (combustible) systems must be present in proportion to one another such that they will react exothermically and become self-sustaining when the composition is ignited at a temperature between 150 C and 1250 C.
The Sekhar invention discussed above, for example, does not appear to teach or suggest the use of a reactive system comprising Al and B2O3 reactive powders for the production of resistive heating elements, and is limited to reactive (combustible) systems that can sustain rapid micropyretic or Self-propagating High-temperature (combustion) Synthesis.
The porosity that is generated in combustion synthesized (SHS) products continues to be the most uncontrollable and serious disadvantage. All SHS processes are initiated by the rapid input of energy from an external source such as an electric arc or heated filament, and such an increase must be rapid enough to prevent any significant conversion of the reactants to the products before the ignition temperature is reached (pre-ignition reaction). Unlike in sintering, where the sample shrinks after the process, during SHS the product usually expands. According to published references, combustion synthesized (SHS) monolithic objects and materials have porosities of up to 40.6 vol. %, and may suffer from other limiting factors such as the segregation of reactants at grain boundaries (Subrahmanyam, et al., 1992, above). Due to a lack of control over the pressure-less SHS process, the products have a tendency to develop non-homogeneous phase distributions. For heating elements, the non-uniformities in the products lead to destructive localized overheating in areas where the non-uniformities exist. It is clear that components and materials produced with the pressure-less SHS process have degraded properties, and can be characterized as mechanically weak and fragile. Although much prior work has centered on developing techniques to eliminate porosity in combustion synthesized (SHS) materials, success has only been achieved in a very limited number of cases. Combustion synthesis processes are rapid by nature, and the time for simultaneous sintering is too short to be of value. External force or pressure is difficult to apply in most situations involving combustion, and the application of pressure limits this method to use with simple shapes like cylinders, and to situations where die damage is not a problem. If a liquid phase is involved then it is hoped that this liquid phase will wet the products and will fill the porosity that is formed during the combustion reaction, creating a denser, stronger product. Unfortunately, most often the liquid phase does not wet the products, or the volume of the liquid may not be enough to fill the pores and the residence time of the hot liquid may be too short to fill the pores in time. In U.S. Pat. No. 5,837,632 Sekhar, et al., claim porosity in certain combustion synthesized heating elements may be significantly reduced by applying a large electrical current (100 Amps or more) and heating the elements up to high temperatures (1500–1600 C) for extended times, however it appears that excessive shrinkage rates in the length (about 28%) and volume (about 48%) must take place in order to sinter the element. The large shrinkage rates and high temperatures probably make this densification technique impractical for many important applications (such as the conformal lamination of combustion synthesized SHS materials, or co-lamination with other materials and substrates) that will be described in the present invention.
Other deposition technologies include arc plasma or flame spraying methods that utilize a very expensive, high-energy beam machine capable of producing ceramic coatings on suitable substrates. Only materials that due not sublime or dissociate at high heat are suitable for arc plasma spraying. The plasma sprayed ceramic coatings are brittle and tend to have a high degree of undesirable as-sprayed porosity. Also, the process is not well suited for the fabrication of precision miniaturized “meso-scale” devices.